Fit rigid (as opposed to mesh) escape panels/windows to a trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 1
Background information and definitions
Modifying fishing trawl nets is a common practice to improve selectivity, helping fishers to catch only the species and sizes they are commercially targeting. How trawl nets are modified depends largely on the sizes/shapes of the unwanted species and also behavioural characteristics. For example, the avoidance response of some species to trawl gear is to swim upwards, while some may attempt to flee in other directions. The use of panels or windows of modified netting (different orientation and/or mesh size) in trawl net codends has been assessed as one means to allow the escape of unwanted fish (see ‘Fishing gear modification – Fit mesh escape panels/windows to a trawl net’). Rigid escape windows are intended to work on the same principle as mesh escape windows but are specifically designed to enable the potential escape of flatfish species. Unlike mesh windows, rigid escape windows are made of steel and have a grid-like construction that creates well defined rectangular escape openings. They are located with the aim that unwanted flatfish (plus small or undersized roundfish species) can pass through the window in a natural swimming orientation and escape.
Supporting evidence from individual studies
A replicated, paired, controlled study in 2013 of an area of seabed in the western Baltic Sea (Santos et al. 2016) found that a trawl net modified with rigid escape windows in a section of net mounted in front of the codend, reduced the catches of unwanted flatfish compared to a trawl net without rigid escape windows. Total catches of plaice Pleuronectes platessa were 56% lower and flounder Platichthys flesus 62% lower in the modified trawl compared to standard trawl (windows: 1,033–1,310 fish, no windows: 2,354–3,437 fish). There was no significant difference in overall catches of the commercial target species, cod Gadus morhua, in the modified and standard trawls (windows: 1,602 fish, no windows: 1,824 fish) or of undersized cod (windows: 255 fish, no windows: 377). Catch comparison trials were done in March 2013 on a commercial twin trawler on cod fishing grounds west of the island of Bornholm. A total of 12 paired trawl deployments were completed using one modified trawl and one standard trawl net. Both nets were identical and fitted with a mandatory design of selective codend of large square mesh (“Bacoma”). In the modified trawl, rigid grid-like escape windows of 38 mm horizontal bar spacing were incorporated in the two side panels of a four-panel net extension piece in front of the codend (“FRESWIND” system - see paper for specifications). Catches from each haul were weighed by species, and the total length of all fish measured.Study and other actions tested